中四物理放大鏡一問..

普通一塊放大鏡既焦距係唔係好大?
因為我渣住塊放大鏡拉到ok遠都仲可以見到物體放大

The focal length of a magnifying glass is about 15~20 cm. This is for the convenience of using the magnifying glass to read printed materials like books and newspapers.
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咁係唔係代表d物體係F之內?

Yes. When used as a magnifying glass, the object must be located between the focus and the lens. Otherwise, the image that you see will be inverted.
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但當我真係拉到好遠既時候..像就變到好模糊..咁係咩原因?

The object is located at the focus of the lens at the situation that you described. The image is formed at infinity
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仲有一個問題就係..物體係焦點..像就變到無限遠..無限遠即係咩呀?

In real practice, when the distance of the object from the lens is large compared with the focal length of the lens, the object can be regarded as at infinity.

For example, if an object is at a distance of 100f (i.e. 100 times the focal length of the lens) in front of the lens, the image is formed at 0.99f behind the lens. Hence, to a good approximation, the image can be treated as at the focus of the lens (for a magnifying glass of focal length 10 cm, the difference is only 1 mm, about 1%).

Therefore, the term [at infinity] can be interpretated, in the actual practice, as at large distance from the lens when compared with the focal length of the lens.
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既然變得無限遠..點解d人影相成日都話對準焦點?

To take a sharp and clear photograph, the image must be formed at the focus of the lens.

Since the object (person or scene) to be photographed is usually at large distance from the lens (typical small digital camera has a lens focal length of 35mm, thus a person standing at 2 metres away, which is already 57 times the focal length, can be regarded as at infinity), thus the image is more or less formed at the focus of the lens.

The term [對準焦點] actually means to set the image, not the object, be formed at the focus.

if you hold a magnifying lens near a window,
you can find an inverted image behind the lens.
the distance between the lens and the image is the focal
length of the lens.
since the camera is usually auto focus, we need not
care about the problem of focusing.
"對準焦點" means looking at the camera.
it does not mean that the object is in the focal length of
the lens.